This invention relates generally to metal oxide, nonlinear voltage variable resistors. More particularly, the invention relates to metal oxide varistors having breakdown electric fields greater than 1200 volts per millimeter at 1 milliampere per square centimeter current density.
There are a few known materials which exhibit nonlinear resistance characteristics and which require resort to the following equation to quantitatively relate voltage and current: EQU I=(V/C).sup..alpha.
where V is the voltage between two points separated by a body of the material under consideration, I is the current flowing between points, C is a constant, and .alpha. is an exponent greater than 1. Both C and .alpha. are functions of the composition and process parameters utilized in the formation of the material. Materials such as silicon carbide exhibit nonlinear exponential resistance characteristics and are utilized as commercial varistors, however, such nonmetallic varistors generally exhibit an .alpha. exponent of no more than 6 and are, therefore, unsuitable for many voltage regulation and transient suppression applications.
A new family of varistor materials having .alpha. exponents in excess of 10 within the current density range of 10.sup.-3 to 10.sup.2 amp.multidot.cm.sup.-2 have recently been produced by sintering a mixture containing a major portion of zinc oxide with bismuth oxide and other metal oxides and/or halides. Silicon dioxide has, occasionally, been utilized as an additive, in concentrations of 20 mol percent or less, in the aforementioned zinc oxide-bismuth oxide based varistor materials. However, these compositions exhibited relatively low breakdown fields and .alpha. exponents less than 6, which are not generally considered to be useful for high voltage regulation applications. U.S. Pat. No. 3,764,566 to Matsuoka et al and U.S. Pat. No. 3,928,245 to Fishman et al describe metal oxide varistors which comprise zinc oxide, bismuth oxide, and silicon dioxide. A comprehensive list of patents and technical publications which describe methods, formulas, and additives useful in the production of zinc oxide based varistors is included in the aforementioned Fishman et al patent. Those patents and technical publications are incorporated herein, by reference, as background material.
The breakdown field of a varistor material may be defined as the electric field which must be applied to a body of the material in order to produce a current density of 10.sup.-3 amp.multidot.cm.sup.-2 in the material. Prior art metal oxide varistor materials generally exhibit breakdown fields between approximately 30 volts per millimeter and approximately 1200 volts per millimeter.
It is desirable, however, to produce varistor components having breakdown fields in excess of 1200 volts per millimeter with exponents in excess of 10.